Inorganic pigments which have been found most suitable for incorporation in coating compositions for producing ultra-lightweight coated papers are minerals of the layer lattice silicate type of which kaolin clay is the most widely used, but coating compositions may also comprise other layer lattice silicate minerals such as talc. These layer lattice silicate minerals generally crystallise in the form of flat plates which may be relatively easily cleaved apart along a plane parallel to the face of the plates. If the coating applied to the surface of a sheet of cellulosic material is light in weight, it follows that the thickness of the dry coating will be small. It has been found that, if a coating is to possess good opacity, gloss and printing properties, the particles of the layer lattice silicate mineral in the coating composition should generally be of high aspect ratio, in other words the ratio of the longest dimension measured across the face of the particle to the thickness of the particle should be large, and the particles should be generally oriented with the plane of the plates parallel to the surface of the cellulosic material. Layer lattice silicate minerals generally exist in the form of particles consisting of stacks or clusters of plate-like crystals and the aspect ratio of the particles of such a mineral may be increased by cleaving apart the plates to provide a particulate material which consists predominantly of individual plates.
However, paper coating compositions containing a high proportion of particles of high aspect ratio suffer from the disadvantage that their rheological properties are generally poor, i.e. a paper coating composition containing a high percentage by weight of such particles tends to be highly viscous and to exhibit rheological dilatancy, i.e. the viscosity of the composition increases with the rate of shear applied to the composition. One way of overcoming this disadvantage would be to reduce the percentage by weight of particulate silicate mineral in the paper coating composition but such a measure would introduce other disadvantages, viz:
1. the cost of drying the coated material would be increased because the quantity of water to be evaporated per unit weight of pigment applied is increased;
2. the aqueous medium would tend to migrate rapidly into the cellulosic sheet material leaving the platelet particles immobilised in an orientation which may be far removed from the optimum orientation in which the plane of the plates are parallel to the surface of the cellulosic material; and
3. the particles would tend to collapse in random orientation into relatively large holes or fissures between fibres of the cellulosic sheet material with the result that the upper surface of the coating, when dried, would not be sufficiently smooth to receive a good print impression.
It is therefore desirable to use a coating composition containing the highest possible percentage by weight of particulate silicate mineral of high aspect ratio to produce an ultra-lightweight coated paper.
A widely used method of coating webs of cellulosic material utilises apparatus which includes a spring steel blade which in use extends across the web of cellulosic material to be coated and is biased into contact with the web, the web being generally supported on a slightly resilient curved surface, such as a roll faced with an elastomeric material. The blade generally makes an acute angle with the tangent to the curved supporting surface along the line of contact of the blade with the surface and is in a trailing attitude with respect to the direction of motion of the web. For obvious reasons such a blade is known as a "trailing blade" and an apparatus using such a blade is generally known as a "trailing blade paper coating apparatus". In one method of coating cellulosic material using a trailing blade paper coating apparatus an aqueous coating composition is introduced into a trough of which the floor and back are formed by the blade and its supporting structure, the sides are formed by suitable dams, and the front of the trough is closed by the web of cellulosic material on the curved support. In another method, an aqueous coating composition is applied to the web upstream of the trailing blade by a suitable applicator, such as a rotating roll or brush in contact with the moving web or by means of spray jets. In either method the trailing blade serves to remove surplus coating composition and to smooth and level the coating. If a coating composition, which is supported on a moving web of cellulosic material and contains a high percentage by weight of particles which are predominantly of high aspect ratio, is constrained to pass at high velocity beneath a trailing blade, the coating is suddenly exposed to conditions of very high shear and, as a result of the poor rheological properties of the coating composition (by virtue of the random orientation of particles of high aspect ratio), there tend to occur sudden changes in the velocity of flow of the composition beneath the blade and in the clearance between the blade and the web with the result that the coating is unevenly applied.
It is an object of the present invention to provide an apparatus for, and a method of, applying to a cellulosic sheet material a substantially smooth and level coating, even at a small thickness, of a composition which contains a relatively high percentage by weight of particles which are predominantly of high aspect ratio.